Development of ITS (Intelligent Transport System) is being advanced with a view to reducing traffic accidents. An example of the ITS is an inter-vehicle communication system in which each vehicle equipped with a communication device broadcasts information about the vehicle running conditions and the like to other vehicles existing within a radius of several hundred meters.
Meanwhile, in recent years, development of WiMAX (Worldwide Interoperability for Microwave Access), which adopts an OFDMA (Orthogonal Frequency Division Multiple Access) scheme for flexible multiplexing of data in directions of both the frequency and time axes, has also been promoted to provide medium-range, large-capacity wireless communication systems (see, e.g., IEEE 802.16-2004, IEEE 802.16e-2005, and IEEE 802.16-2004/Corl-2005). In ITS, a vehicle-mounted communication device occasionally transmits large-capacity data such as image data, and thus, application of WiMAX systems to inter-vehicle communication is being considered. For example, a vehicle is mounted with a WiMAX base station as a transmitter and also with a WiMAX terminal as a receiver, to enable inter-vehicle communication.
In WiMAX, the base station transmits, to each terminal, a radio frame including a preamble for synchronizing the radio frame, a frame control header (FCH) indicating the size and the like of region (burst) allocation information (map) in the radio frame, and the region (burst) allocation information (map). Using the bursts in the radio frame allocated to the individual terminals, the base station communicates data with the terminals. Namely, the burst is allocated for each communication connection. In the following description, it is assumed that the radio frame is absolutely synchronized by means of the GPS (Global Positioning System) or the like, making it unnecessary to carry out a synchronization acquisition process using the preamble.
Where the communication scheme is broadcast, a burst defined as a multicast connection is received by all terminals. In this case, if radio frames transmitted from vehicles (as base stations) are different in map content, then the map information in one radio frame interferes with that in another, with the result that the terminals fail to locate the bursts and thus are unable to receive data. This arises because the map start position is fixed within the radio frame.
FIG. 7 illustrates a radio frame structure. A radio frame comprises the preamble, the FCH, a DL (Down Link)-MAP, a UL (Up Link)-MAP, DL bursts 1 to 15, and UL bursts 1, 2, . . . . The preamble, the position of the FCH, and the start position of the DL-MAP are fixed. The UL-MAP starts following the end position of the DL-MAP. According to the IEEE 802.16e standards, the DL-MAP and the UL-MAP contain information about the connection IDs of respective bursts, burst start positions, burst end positions, and burst sizes. The information other than the UL bursts 1, 2, . . . is generated by the base station.
Thus, data is transmitted in the bursts from multiple vehicles via multicast connections, and if a DL-MAP differs in content from another, the DL-MAPs interfere with each other because the start positions of the DL-MAPs are fixed, making the terminals unable to receive the data.
Accordingly, in order to prevent the interference of DL-MAPs, radio frames need to be transmitted in such a manner that a single base station mounted on a vehicle is allowed to transmit a radio frame at a time. The length of the radio frame used in WiMAX is generally 5 ms. Thus, where each vehicle transmits information at intervals of 1 s, a total of 200 vehicles in the radio wave coverage area (area with a radius of several hundred meters) can transmit information.
In urban districts, however, a large number of vehicles exist, and a few hundred is not sufficient for the number of vehicles that are allowed to communicate within the radio wave coverage area.
Also, when information indicating the structure of a radio frame is transmitted from multiple communication devices, there is a possibility that communication failure occurs due to interference.
Further, there is another problem that while transmitting information indicative of the radio frame structure, the communication device is unable to receive information indicative of the radio frame structure.